1. Field of the Invention
This invention relates to a digitally controlled adaptive power supply interface for adjusting a system supply voltage in response to changes in the system operating frequency, and more particularly to an improved digital adaptive power supply interface with self calibrated analog feedback and with control synchronized to the system operating frequency.
2. Description of the Prior Art
As will be appreciated by those skilled in the art, the power consumed by a synchronous digital system, such as a processor for example, is proportional to its operating frequency. Significant power and energy savings are possible if the supply voltage to the digital system is adjusted to match the system operating frequency.
A paper entitled A Fully Digital, Energy-Efficient, Adaptive Power-Supply Regulator by Wei and Horowitz published in the IEEE Journal of Solid-State Circuits, Vol. 34, No. 4, April 1999, incorporated herein by reference, describes a prior art digitally controlled adaptive power supply. FIG. 1 of this application is based on FIG. 8 of the paper and some of the basics of the prior teaching will be summarized here for convenience with reference to FIG. 1.
A reference signal fref indicative of the digital circuit operating frequency and the output fosc of a voltage controlled oscillator VCO, indicative of the output voltage Vo, both feed counters and the number of transitions are counted for a fixed period of time. The difference between the outputs of the two counters corresponds to an error signal that is a function of the output voltage Vo and the desired voltage for a digital system operating at the frequency fref. The binary equivalent of the error between the reference and oscillator frequencies feeds into a digital Proportional Integral Derivative (PID) controller, which uses the error value to make appropriate corrections to the output. The output of a pulse width modulated DC/DC converter is therefore adaptively regulated to the required voltage for the digital circuit operating frequency. A system clock floop clocks the digital PID controller and sets the time base for the overall system. Over a period of floop the counters count the reference and oscillator clock pulses to generate the equivalent numeric representations of the frequency, and it is over this same period that the variable duty cycle of the DC/DC converter is switched.
The VCO provides a simple analog to digital conversion, but the digital conversion value for a given voltage input varies with temperature and semiconductor process variations used in making the VCO. This is a source of error in the prior art systems. Further, the prior art adaptive power supplies are difficult to synchronize to the system they are supplying and do not provide a simple processor interface for accurate adjustment of the supply voltage by a processor.